1. Field of the Invention
This invention relates to radar for detecting vehicles on a roadway, and more particularly to a side-looking radar system and method for distinguishing objects in a host vehicle's blind spot that are moving along with the host vehicle from objects that pass through the blind spot and are stationary with respect to earth.
2. Description of Related Art
It is critical for the safe operation of a motor vehicle that the operator of the vehicle be able to detect obstacles that are near the vehicle, but which are not easily directly observable from the driver's seat. Regions in which objects that are near a vehicle are not directly observable are commonly referred to as "blind spots". For example, the region between the angles of 90.degree. and 170.degree. measured with respect to the forward direction of a vehicle in a clockwise direction (i.e., generally to the right the vehicle and slightly behind the operator's seat) is typically a blind spot, particularly for large vehicles such as buses and trucks. Failure of an operator to be aware of an object (typically another vehicle) in this right-side blind spot when making a right-hand turn or a right lane change is a source of numerous accidents. Another common blind spot is the region directly to the rear of a vehicle. This region is of particular interest when the vehicle is in reverse gear (i.e., when "backing up").
The most common solution to the problem of blind spots has been to use mirrors to aid the operator of the vehicle in detecting the presence of obstacles that could pose a hazard. Such mirrors have been made in a variety of shapes and having a variety of lenses. In addition such mirrors have been mounted in various locations to provide the operator with the greatest ability to detect the presence of obstacles in particular blind spots. For example, concave mirrors are commonly mounted to the right side of a vehicle and aimed at the right-side blind spot. Mirrors provide the operator with some information regarding the presence of obstacles in a vehicle's blind spots. However, mirrors are less useful at night and under adverse weather conditions. Even in the best of conditions, mirrors that distort the reflection are typically required to allow the operator to view the right rear blind spot. Some operators find it difficult to properly interpret the image that is presented in such mirrors (such as convex mirrors that are commonly used as right side mirrors). Furthermore, mirrors tend to reflect the headlights of vehicles approaching from behind and thus blind the driver of the vehicle on which the mirror is fixed. Hence, a more complete and satisfactory solution is still sought by many.
One known alternative to the use of mirrors to detect obstacles in a vehicle's blind spot is to mount a camera on the vehicle to provide the operator with a visual image of obstacles in the vehicle's blind spot. However, this solution is complex and expensive, requiring a video camera and video monitor. Further, a video monitor can present a complex image, which, even if undistorted, can be difficult to interpret rapidly under stressful conditions that occur during heavy traffic conditions. Still further, monitors can be distracting. Moreover, like mirrors, such camera systems are less useful at night and under adverse weather conditions.
Another alternative to the use of mirrors is to direct radar transmissions toward each blind spot. Reflections of the radar transmissions can then be detected to determine the presence of obstacles in each of the blind spots. One such system is disclosed in U.S. Pat. No. 5,325,096, issued to Alan Packett and assigned to the owner of the present invention. These systems use a common radar transceiver that transmits a radio frequency (RF) signal into a blind spot of the vehicle. Obstacles that are present in that blind spot region reflect the transmitted signal. The frequency of the transmitted signal is compared with the frequency of a reflection of the transmitted signal which is received within the radar system to determine whether the reflected signal has been Doppler shifted. A Doppler shift in the frequency generally indicates that an obstacle is present in the blind spot.
However, one objection to such Doppler radar blind spot sensors is that objects that are stationary on the side of the road, such as road signs, guard rails, parked vehicles, etc. cause a "nuisance condition". That is, objects that the host vehicle (i.e, the vehicle equipped with the radar system) passes will cause the radar system to indicate the presence of these stationary objects in the host vehicle's blind spot even though they are not considered a threat. This can influence the operator of the host vehicle to lose faith in the usefulness of the radar and reduce the system effectiveness for warning the operator of real threats. In addition, such indications are distracting and irritating to the operator.
Accordingly, there is presently a desire for a simple and inexpensive solution to the problem of detecting hazardous obstacles in the blind spots of a vehicle. Such a solution should also be useful at night and under adverse weather conditions and should not generate nuisance conditions in response to objects that are stationary at the side of the road as the host vehicle passes. The present invention provides such a solution.